1. Field of the Invention
This invention relates to regeneration of binary data signals and more specifically to a forward error correction (FEC) assisted receiver optimization.
2. Background Art
It is well known that signals suffer degradation between the transmitter and receiver from sources related to sampling and quantizing effects, and channel effects.
The sampling and quantizing effects comprise the distortion inherent in quantization, which could be a round-off or truncation error, errors introduced by the quantizer saturation, and timing jitters. Generally, saturation may be avoided by using automatic gain control (AGC), which extends the operating range of the quantizer. Jitters are any deviation of the sample of the input signal from its designated position, and its effect is equivalent to a frequency modulation. Timing jitter is generally controlled with very good power supply isolation and stable clock references.
The corruption introduced by the channel is due to such factors as noise, inter-symbol interference, dispersion, etc. The degradation of the recovered signal quality with the channel induced errors is called "threshold effect".
If the channel noise is small, there will be no problem detecting the presence of a waveform, the only errors present in the reconstruction being the sampling and quantizing noise. On the other hand, if the channel noise is large, the resultant detection errors cause reconstruction errors. Thermal noise, interference from other users, and interference from circuit switching transients can cause errors in detecting the pulses carrying the digitized samples.
Intersymbol interference is due to the bandwidth of the channel. A band-limited channel spreads the pulses, and if the width of the pulse exceeds a symbol duration, overlap with neighbouring pulses may occur.
Dispersion is the chromatic or wavelength dependence of a parameter, as for example the distortion caused by different wavelengths of light within the pulse, travelling at different speeds. The pulse distortion in a fiber optic system may, for example, be caused by some parts of the light pulses following longer paths (modes) than other parts.
The degradation of a signal is expressed in BER (bit error rate) which is the ratio between the number of erroneous bits counted at a site of interest over the total number of bits received.
In the last decade, transmission rates of data signals have increased very fast. For high rate transmission, such as at 10 or 40 Gb/s, signal corruption introduced by the transmission channel is a critical parameter. The demand for receivers with high sensitivity increased progressively with the transmission rates. The receiver's task is to decide which symbol was actually transmitted. For a given BER, the system performance is dependent upon the decision level, defined also as threshold level or as a slicing level, which is used for data regeneration. For example, a threshold level variation of only 8% can result in a variation of the receiver sensitivity of up to about 1 dB. Detection errors may develop as a result of an incorrect decision level or incorrect clock/data timing being selected.
Current optical receivers comprise an avalanche photodiode (APD), or a high performance PIN photodiode, coupled to a transimpedance amplifier. The transimpedance amplifier is a shunt feedback amplifier acting as a current-to-voltage transducer. The signal is then amplified and a data regenerator extracts the information from the amplified signal. Generally, binary data regenerators are provided with a fixed threshold level selected such as to provide the best error rate at a predetermined signal power level. However, a fixed threshold cannot account for the effects of aging of the components, temperature variations, etc. As a result, higher power levels need to be transmitted to account for the above factors, which in turn diminish the length of the transmission channel.
As the requirement for essentially error free operation for fiber systems became more stringent, systems which allowed errors to occur during the normal data regeneration mode of operation are currently less acceptable. Driven by customer demand, sophisticated performance monitors are provided at the receiver site, which perform optimization routines for lowering the BER of the recovered signal.
It is known to generate a control code at the transmission site which is then transmitted with the information along the communication link. Error detection is based in general on comparison between the transmitted and the received control code. Error correction is based on various algorithms which compensate for the specific error detected in the control code. This method is known as forward error correction (FEC).
A data regenerator including a performance monitor is disclosed in U.S. Pat. No. 4,097,697 (Harman, issued on Jun. 27, 1978 and assigned to Northern Telecom Limited). This patent discloses a first differential amplifier which regenerates the data signal by comparing the incoming signal with a fixed threshold. A second differential amplifier compares the incoming signal with an offset slicing level to produce an error-ed regenerated signal. Both differential amplifiers are clocked by the recovered clock signal. The regenerated signals are compared to each other and the result is used to determine the degradation of the incoming signal.
U.S. Pat. No. 4,799,790 (Tsukamoto et al., issued Jan. 24, 1989 and assigned to Anritsu Corporation) discloses a device comprising a transmitter for launching signals of various wavelengths into a reference or test fiber, and a receiver. At the receiver, the phase difference between two adjacent wavelengths is measured for both the reference and test path for determining the delay of the respective wavelength.
None of the above patents is concerned, however, with providing a simple device and method for detecting and correcting errors in the recovered signal which uses the information in the data path itself. The receiver circuits described in the above patents rely on duplicate channels and pseudo-error detection. The prior art error detecting circuits must be located at the receiver site, which results in a reduced flexibility of the system architecture.
The extent of signal degradations may be directly measured using an eye closure diagram, which is the graphic pattern produced on an oscilloscope when a baseband signal is applied to the vertical input of the oscilloscope and the symbol rate triggers the instrument time base. For a binary signal, such an eye diagram has a single eye which is open or closed to an extent determined by the signal degradation. An open pattern is desired. Changes in the eye size indicate intersymbol interference, amplitude irregularities, or timing problems.
U.S. Pat. No. 4,823,360 (Tremblay et al., issued Apr.; 18, 1989 and assigned to Northern Telecom Limited) discloses a device for measuring chromatic dispersion of an optical fiber based on a baseband phase comparison method, using the eye closure diagram of the signal received over the transmission link. The device described in this U.S. patent evaluates the transmission link performance using three threshold levels for recovering data. Two of the thresholds are obtained by measuring on the eye diagram the level of "long 0s" and "long 1s", respectively, for a preset error rate, and the third threshold is provided in a selected relationship to the other two to produce regenerated signals.
The technique described in the '360 patent is based on generating "pseudo-errors" on separate pseudo-error channels. The pseudo-errors give some idea of how error performance varies with the slicing level and, because they do not appear on the in-service transmission path, they do not affect service. Consequently, this technique can be used for dynamic control of in-service systems. Unfortunately, the separate pseudo-channels require additional high speed circuitry, and the pseudo-errors may not give a true reflection of error performance.